1. Field of the Invention
This invention relates to radiant energy sensors, and more particularly to an improved radiant energy sensor employing heavily doped layers within a crystalline substrate to reduce optical reflections and blooming.
2. Description of the Prior Art
In the prior art, a radiant energy sensor may be formed by fabricating a plurality of detectors on a crystalline substrate. Normally, the radiant energy sensor would have the radiant energy from a field of view focused on a plurality of detectors to convert the radiant energy into electrical signals. The optics or lens system which focuses radiant energy from a field of view onto the plurality of detectors or focal plane surface causes the radiant energy to diverge as it passes beyond the focal point or focal plane into the crystalline substrate. If the radiant energy is not totally absorbed in the detector, radiant energy will pass through the detector and exit the crystalline substrate or be reflected back into the detector by the lower side of the crystal substrate. If the radiant energy will remain in the same detector, the sensitivity of the detector would increase. If however the radiant energy diverges outside its detector into an adjacent detector, then the adjacent detector sensitivity is degraded due to absorbing radiant energy not originally focused on it. For any reasonable lens f/number used in the lens system for focusing such as f/2, the radiant energy or reflected rays diverge as they propagate. The phenomenon of radiant energy passing through its intended detector and then being reflected into an adjacent detector is known as optical crosstalk.
The problem of optical crosstalk is especially acute in extrinsic silicon infrared photodetectors because they have a lower optical absorption coefficient than intrinsic silicon infrared photodetectors or visible photodetectors.
One method for reducing optical reflection is to apply an anti-reflection film on the back surface of the crystalline substrate. However, the anti-reflection film must have an index of refraction suitable for the material of the crystalline substrate over a broad wavelength range to provide broad wavelength anti-reflection performance. While anti-reflection films or quarter wave plates can be provided, their performance on a silicon substrate is not uniform, even in the wavelength range from three to five micrometers, for example.
In the prior art, the concentration of the dopants in crystalline substrates has been measured by determining the reflectivity of the doped layers such as described in an article by L. E. Howarth and J. F. Gilbert entitled "Determination of Free Electron Effective Mass of N-Type Silicon", published in Journal of Applied Physics, Vol. 34, pp. b 236-237, January 1963. The amount of radiant energy reflected by a doped layer was in part due to free carrier absorption in the doped layer.
It is desirable to reduce the optical crosstalk in certain radiant energy sensors by providing a means for absorbing the radiant energy after it passes through the predominant portion of a detector to prevent its reflection into adjacent detectors.